The present invention relates to display apparatus, and more particularly to display for enhanced radar image interpretation.
Radar images can be fed to a cathode ray tube display for display, e.g., to an aircraft crew member. Synthetic aperture radar (SAR) images are often stored in log-detected format. Direct display of the logarithm of SAR image intensity expands the dynamic range of the background terrain, but compresses the dynamic range of bright discretes.
One problem in image interpretation of the displayed imagery is to quickly locate or distinguish targets. One known technique for enhancing the radar image interpretation is grey level intensity remapping ("GLIR").
GLIR is a function applied to image intensity. Intensity remapping can enhance visual target detection making it less time consuming. FIG. 1 shows a plot of an exemplary GLIR function. In FIG. 1, T is the threshold value and S.sub.1 is the intensity where the slope of the GLIR function is one. Intensities between zero and S.sub.1 are compressed into a smaller dynamic range. Intensities between S.sub.1 and T are expanded into a larger dynamic range. Any intensity greater than T is saturated into the maximum intensity value (255 for eight-bit pixels). The intensity values between zero and S.sub.1 are assumed to be background. To gain dynamic range for target detection some background detail (context) is sacrificed. The intensity values between T and 255 are assumed to be targets. All target detail is discarded to gain greater dynamic range for target detection. The pixels with intensity values between S.sub.1 and T are possible targets. The dynamic range of this region is expanded so that the targets can be found more quickly.
Another interpretation enhancement technique is pseudo-coloring or color segmentation. For a reference describing several monochromatic and pseudo-coloring techniques, see "Synthetic Aperture Radar Image Processing Techniques Development and Evaluation," Final Report Number AFAL-TR-77-113, prepared by Hughes Aircraft Company for the Air Force Avionic Laboratory, Wright Patterson Air Force Base, Ohio. Pseudo-coloring remaps each pixel intensity into, for example, three new intensity values. These three remapped values are used to control the intensity of the color guns (one red, one green, one blue) of a color cathode ray tube (CRT).
A primary goal in using pseudo-color techniques is to enhance visual target detection without losing image detail. Visual target detection enhancement is achieved by color transitions. For example, in some known pseudo-color techniques, green is selected for low intensity returns (background), yellow for medium intensity returns (possible targets), and white for high intensity returns (targets and corner reflectors). Most of the image is low intensity returns. Green is typically selected for this region because visual detection thresholds are color dependent; the human visual system is more sensitive to green. For a reference discussing this phenomenon, see "Digital Picture Processing," A. Rosenfeld et al., Academic Press, Inc., 1982, second edition, volume 1, at page 62. Yellow is typically selected for medium intensity returns to create a green-phosphor display effect (some earlier green phosphor CRT display turned yellow at high intensities). White is typically selected for very high intensity returns to identify corner reflectors and very high intensity targets. Given the choice of color combinations, various mappings from pixel intensity to the desired color set have been employed.
Two common color segmentation algorithms are shown in FIGS. 2 and 3. FIG. 2 shows a pseudo-color remapping that gives the maximum expansion of the dynamic range. This remapping creates artificial color boundaries and artificial intensity boundaries at the intensity thresholds at T1 and T2. Artificial boundaries are created by discontinuous changes in color and intensity. The remapping shown in FIG. 3 also produces artificial color boundaries, but not artificial intensity boundaries. These abrupt changes in color and intensity distract the viewer from the real targets.
It would therefore represent an advance in the art to provide a pseudo-color remapping that would make continuous changes in intensity and color.
It is a general object of the invention to provide a pseudo-color display which enhances the visual detection of targets.
A further object is to provide a pseudo-color display which provides gradual color and intensity transitions.
Additional objects of the invention are to provide a pseudo-color display employing a green-yellow-white color remapping, using green as the primary (background) color, which requires only one intensity threshold and retains the image background.